Wide band multichannel electron multiplier having improved path shielding and gain characteristics



R. KALIBJIAN 3,229,142 WIDE BAND MULTICHANNEL ELECTRON MULTIPLIER HAVING IMPROVED PATH SHIELDING AND GAIN CHARACTERISTICS Jan. 11, 1966 Filed Oct. 22, 1962 UUUUUUUUUU INVENTOR.

RALPH KAL/BJ/A/V ATTORNEY.

United States Patent 3,229,142 WIDE BAND MULTKCHANNEL ELECTRON MULTIPLEER HAVING lll lPRGVED PATH SHIELDING AND GAEN CHARAETERISTICS Ralph Kalibjian, Liverrnore, Califi, assignor to the United States of America as represented by the United States Atomic Energy Qonimission Filed Get. 22, 1962, Ser. No. 232,317 4 Claims. (6!. 313-68) The present invention relates generally to electron m-ultipliers and, in particular, to an improved channeled electron multiplier assembly.

Channeled electron multipliers are used for a variety of purposes such as quantization of electrical signals, coding, and intensification of images. In many applications where channeled electron multipliers are used, a system is needed that has high gain, good transit-time characteristics, and a minimum of crosstalk between channels.

While various species of channeled electron multipliers have been developed in the past, in general, the versatility of these species has been limited by either low gain, considerable channel crosstalk, or limited bandwidth characteristics. For an example, major difiiculty is encountered in the design of high-gain, multidynode, mulichanneled, electron multiplier tubes in the elimination of crosstalk between channels. High-gain tubes generally require many stages of multiplication. Since in channeled electron multiplier tubes, as known in the art, the problem of eliminating crosstalk between channels has not been solved, as the number of stages of multiplication is increased, the amount of crosstalk between channels is also increased. One method of solving the problem oi eliminating the crosstalk between channels in high-gain, channeled multiplier tubes is to develop a tube requiring less tube gain, viz., a tube using a large beam current. However, tubes using large beam currents are not compatible with the requirement of a wide-bandwidth, deflection system, for example, as found in commercially available traveling-wave deflector electron guns.

In some applications of channeled electron-multiplier tubes, transit-time characteristics become an important consideration. For example, in those tubes employing Venetian-blind dynode arrangements, the electric field established in the multiplication region is relatively Weak. A stronger electric field would shorten the time required for an electron to traverse the multiplying region, thus reducing transit-time dispersion. Also, if the read-out time of the information collected at the collector of a Venetian-blind dynode multiplier is short, in the case of a five-stage multiplier tube less than 2 nanoseconds, the output signal will be a distorted representation of the input signal.

The present invention provides an apparatus which overcomes the limitations of prior-art channeled electronmultiplier systems. More particularly, the present invention comprises a channeled electron multiplier having a Venetian-blind dynode configuration wherein each channcl is defined by a pair of louvers on a series of cascaded dynodes. Shields defining a recess are electrically connected to each dynode such that the free edge of the louver of the preceding dynode extends into the recess of the shield to prevent crosstalk between channels and enhance the electric field of the multiplier. The enhanced electric field results in reducing the electron transit time through the multiplying region, thus minimizing transit-time dispersion.

Accordingly, it is an object of this invention to provide an improved channeled electron multiplier wherein the various channels are isolated from one another to minimize the crosstalk between channels.

Further, it is an object of this invention to provide an improved channeled electron multiplier wherein the transit-time characteristics are improved.

Another object of this invention is to provide an improved channeled electron multiplier wherein the secondary emission ratio of each multiplying stage is maximized.

Yet another object of this invention is to provide an improved channeled electron multiplier capable of converting wide bandwidth information from a single channel into multiple channels of lower bandwidth information.

Still another object of this invention is to provide an improved channeled electron multiplier wherein the gain of all channels may be equalized.

The appearance and operation of the channeled electron multiplier system can best be described with reference to the drawings, in which:

FEGURE l is a schematic view of a cathode-ray switch tube wherein is utilized the channeled electron multiplier of the present invention;

FIGURE 2 is a cross-sectional view of a portion of the channeled electron multiplier of the present invention;

FIGURE 3 is a cross-sectional view of a portion of a single dynode unit; and

FIGURE 4 is a top view of a single dynode further exemplifying the design of the apertures.

Referring to FIGURE 1, there is shown a channeled electron-multiplier assembly 11 within a cathode-ray switch tube 12. The cathode-ray switch tube 12 utilizes, in operation, the usual electron-gun means 13 and a defiection system 14 for accepting the wide bandwidth information, which information provides the modulating signal for the switch tube 12. An electron beam 16 is created by electron-gun means 13 and is deflected as desired by the modulating signal introduced to the deflection system 14. The electron-gun means 13 could be, for example, an electrostatically focused gun with a traveling-wave deflector having a bandwidth of L000 megacycles and a beam current of 15 microamperes, or could be a magnetically focused gun with a deflection bandwidth of 3,600 megacycles and a beam current of 35 microanrperes.

More particularly, the channeled electron-multiplier assembly 11 comprises the combination of aperture-lens assembly 17, a collector assembly 18, and the actual electron multiplier 19. Referring to FIGURE 2, there is shown four channels of a ten-channeled electron-multiplier assembly 11, wherein the aperture-lens assembly 17 comprises a first aperture plate 21 and a second aperture plate 22 in registry with the first plate 21. Sidewalls 23 are placed perpendicular to and between aperture plate 22 and a first dynode 24 (the dynode 24 is also disposed in registry behind the second aperture plate 22) to provide channels generally designated by numeral 26, whereby such walls 23 prevent the marginal rays passing through the apertures within the lens assembly 17 from entering the adjacent channels. The electron multiplier 19 of the chaneled electron-multiplier assembly 11 is formed of a series of Venetian-blind dynodcs, the hereinafter described multiplier is particularly formed of five dynodes (first dynode 24, second, third, fourth, and fifth dynodes 27, 28, 2?, and 31, respectively) placed in cascad-e fashion, one behind the other, with a final collector lens 32 forming the multiplied beam-entrance portion of the collector assembly 18.

Referring to FIGURE 3, each dynode comprises a louvered plate 33 wherein each aperture defines a channel 25. Louvered plate 33 is composed of material having a high ratio of secondary electron emission, such as silver-magnesium alloy. Louver slats 34 are arranged substantially parallel and inclined to the path of the elecplate 37 facing louvered plate 33. Mesh 39 prevents the relatively negative electric field of a preceding dynode from exerting a retarding influence on the secondary electrons liberated. Thus, the collection efficiency of the secondary electrons is increased.

To minimize the number of electrons from beam 16, which could possibly pass directly into the second dynode 27, the first dynode 24 has a shifted configuration to obtain 100% opacity. (A shifted dynode configuration is one where the free edge of louver slat 34 is referenced adjacent to the inner edge of the rib 38 of the same dynode as is shown in FIGURE 2.) Subsequent dynodes 27, 28, 29, and 31 utilize the standard dynode configuration (wherein the free edge of louver slat 34 is referenced away from the inner edge of rib 38 of the same dynode, as in FIGURE 2). A shield 41 defining a recess, a preferred embodiment being a shield having a U shaped cross section, is secured to each rib 38 of dynodes 27, 28, 29, 31 and also each rib 42 defining the apertures in the collector lens 32. Such U shields 41 face and straddle the free edge of the louver slat of the preceding dynode, as shown in FIGURE 2, thereby preventing secondary electrons of high initial energy, or reflected primaries from entering the adjacent channels.

The collector assembly 18 more particularly comprises an array of T shaped collector pins 43, each enclosed in a box shield 44. The entrance of each of the box shields is formed by the apertures in the collector lens 32, of previous mention. The inside walls of the shields 44 are made of silver-magnesium, or the like, in order that they can actually serve as a (sixth) dynode. pins 43 are spaced apart and held in place by a multiformed glass bar 46, and the entire dynode multiplier structure is held in place by multi-formed glass (not shown). A tungsten stocking mesh 47 of low-shadow ratio is spot welded to the collector lens opposite the side upon which the U shields are aflixed.

As can be seen by referring to the example in FIGURE 4, the dynodes 24, 27, 28, 29, 31, collector lens 32, and

-the aperture-plate assembly 17 are generally of circular construction with three fixture post holes 48 spaced evenly about the outer periphery thereof to provide fixture locations for aligning and mounting the multiplier assembly 11 in the cascade configuration of FIGURE 2. The holes 48 are elongated to permit reorientation shifting of the dynodes as necessary for aligning the louvers with the U shields. The dynodes 24, 27, 28, 29, 31 and the collector lens 32 also are provided with rivet holes 49 for individual assembly.

In operation, the electron beam 16, created and accelerated by the electron-gun means 13, is passed through the beam-deflection system 14 to strike the aperture-lens assembly 17. The wide bandwidth information (or modulating signal) introduced to the deflection system 14 controls the sweep of beam 16 and causes same to traverse the array of apertures in the aperture-lens assembly 17. Each aperture in lens assembly 17 is in registry with its respective channel 26 of the electron multiplier 19.

' Each channel is, in turn, also in registry with the cllec tor pin 43 mounted within the box shield 44. Thus, the sequence of electron multiplication of beam 16 passing along a channel, as shown in FIGURE 2, is as follows: Beam 16 passes through the apertures of plates 21 and 22, between side walls 23, through the aperture in dynode 24, through the tungsten stocking mesh 39 to strike the louver slat 34 of dynode 24. Beam 16 is necessarily decelerated to the first dynode 24 in order that the maxi- The collector mum secondary emission can be obtained therefrom. However, the beam of electrons will defocus in traversing through a single aperture because of the retarding electric field. Therefore, the present invention employs the second aperture plate 22, in registry with the first plate 21, at the same potential as that of the first dynode 24 to thereby focus the beam on louver slat 34 of dynode 24. Dynode 24 operates in the manner of all multiplier devices creating secondary emission, the net effect of which is to multiply the impinging electron beam. The multiplied electron beam 16 passes from louver slat 34 of dynode 24 along a predetermined path to strike louver slat 34 of dynode 27. Multiplication again occurs, and the beam is successively passed on from louver slat 34 of dynode 28, louver slat 34 of dynode 29, louver slat 34 of dynode 31, to finally impinge upon the inside walls of the respective collector box shields 44. The potential difference between each of the collector box shields 44 and respective T shaped collector pins 43 is adjusted to equalize the gain of all the channels 26. The secondary electrons liberated at each of the collector box shields 44 are directed and collected by the respective T shaped collector pins 43. It is noted that the number of dynodes or the number of chanels employed is not unique and may vary according to the desired results or application.

In a prototype model of the channeled multiplier of the present invention, typical operating potentials with reference to the cathode potential of the electron gun 13 are: 4,000 volts for plate 21; 1300 volts for plate 22 and dynode 24; and 1750, 2200, 2650, 3100, 3550, and 4,000 volts on dynodes 27, 28, 29, 31, collector lens 32, and collector pins 43, respectively. The U shields 41 secured to ribs 38 are electrically connected to the respective dynodes. As a result of the potential established at the U shields 41, the electric field of the preceding stage of multiplication, e.g., the electric field present in the region bounded by louver slats 34 of dynode 24 and U shields 41 secured to ribs 38 of dynodes 27, is increased. Thus, the transit time required for the secondary electrons liberated from one dynode to pass to the succeeding dynode is greatly reduced. This results in a great reduction in transit-time dispersion (less than 0.9 nanosecond per stage) and, consequently, a channeled electron multiplier employing Venetian-blind dynode configuration having a greater bandwidth.

In theory, amplitude information of the modulating voltage introduced at the deflection system 14 is quantized because the deflector system has a deflection factor of V volts per aperture. Time information or aperture period, T will be determined by the total charge, Q, on the collector (QzCV, Where C and V are respectively the capacitance and voltage on the collector). Since the output multiplied beam current, I is constant, the period, T zQ/l In this manner, the rise portion of the modulating signal is reconstructed in a stairstep fashion by simply knowing the number of channels and the total charge on each channel. The advantage of such a device is that wide bandwidth requirements are confined only to the deflection system 14 of the tube, and not in the output system of the tube, i.e., the multiplier 19 and the collector 18.

While the present invention has been described with respect to a single embodiment, it will be apparent that numerous modifications and variations are possible Within the spirit and scope of the invention and, thus, it is not intended to limit the invention except by the terms of the following claims. 4

What is claimed is:

1. In a Venetian-blind dynode channeled electron multiplier of the class wherein each channel is defined by a pair of louver slats on a series of cascaded louvered dynodes and by an aperture of a collector assembly, a plurality of shields, each of said shields defining a recess, said shields being in electrical conducting relationship to said dynodes and collector assembly, each of said shields being disposed to straddle the free edge of the louver slat of a preceding dynode.

2. A Venetian-blind dynode channeled electron multiplier as claimed in claim 1 further defined by said shields comprising a first extension, a second extension, and a web portion, said first and second extensions integrally connected to opposite ends of said web portion in parallel relationship to each other.

3. In a Venetian-blind dynode channeled electron multiplier of the class wherein each channel is defined by a pair of louver slats on a series of cascaded louvered dynodes and by an aperture of a collector assembly, a first aperture plate having alternate apertures and ribs, a second aperture plate having alternate apertures and ribs disposed behind said first aperture plate, said apertures of said second aperture plate being in registry with the apertures of said first aperture plate, said series of cascaded louvered dynodes being disposed behind and in registry with the apertures of said second aperture plate, side walls secured perpendicularly in electrical conducting relationship to the ribs of said second aperture plate and to the first dynode of said cascaded dynodes thereby defining each channel and collimating the electron beam, a collector lens of alternate apertures and ribs disposed behind the final louvered dynode, said apertures of said collector lens being in registry with the final louvered dynode, a plurality of hollow rectangular housings having five planar sides wherein four of said planar sides define a rectangular aperture, the aperture of each housing being in registry with an aperture of said collector lens, each housing being secured in electrical conducting relationship to the ribs of said collector lens, the inner face of said five planar sides having a secondary emissive surface wherein each housing serves as an additional stage of electron multiplication, a plurality of collector pins, each of said pins penetrating respectively one of said housings through the planar side opposite said rectangular aperture thereof and being insulated therefrom, and a plurality of shields, each of said shields defining a recess, said shields being in electrical conducting relationship to said dynodes and collector assembly, each of said shields being disposed to straddle the free edge of the louver slat of a preceding dynode.

4. A channeled electron multiplier having an electronsource accelerating means, a deflection system, and a generated electron beam, said multiplier comprising a first aperture plate having alternate apertures and ribs, a second aperture plate having alternate apertures and ribs disposed behind said first aperture plate, said apertures of said second aperture plate being in registry with the apertures of said first aperture plate, a first louvered dynode of alternate apertures and ribs disposed behind said second aperture plate, said apertures of said dynode being in registry with the apertures of said aperture plate, side walls secured perpendicularly in electrical conducting relationship to the ribs of said second aperture plate and to the ribs of said first dynode thereby defining each channel and collimating the electron beam, at least a second louvered dynode disposed behind said first dynode, said ribs defining the apertures of said second dynode being disposed directly behind the free edge of the louver slats on the first dynode, a collector lens of alternate apertures and ribs disposed behind the final louvered dynode, said apertures of said collector lens being in reg istry with the final louvered dynode, a plurality of hollow rectangular housings having five planar sides wherein four of said planar sides define a rectangular aperture, the aperture of each housing being in registry with an aperture of said collector lens, each housing being secured in electrical conducting relationship to the ribs of said collector lens, the inner face of said five planar sides having a secondary emissive surface wherein each housing serves as an additional stage of electron multiplication, a plurality of collector pins, each of said pins penetrating respectively one of said housings through the planar side opposite said rectangular aperture thereof and being insulated therefrom, a plurality of shields, each of said shields defining a recess, said shields being in electrical conducting relationship to said dynodes and collector assembly, each of said shields being disposed to straddle the free edge of the louver slat of a preceding dynode, and a stocking mesh mounted across the apertures of the dynode and collector lens.

References Qited by the Examiner UNITED STATES PATENTS 2,591,012 4/ 1952 Salisbury 313- X 2,674,661 4/1954 Law 313-105 X 2,790,922 4/1957 Jenny 313105 2,942,133 6/1960 McGee 313-105 X FOREIGN PATENTS 107,443 6/ 1939 Australia.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

C. O. GARDNER, Assistant Examiner. 

1. IN A VENETIAN-BLIND DYNODE CHANNELED ELECTRON MULTIPLIER OF THE CLASS WHEREIN EACH CHANNEL IS DEFINED BY A PAIR OF LOUVER SLATS ON A SERIES OF CASCADED LOUVERED DYNODES AND BY AN APERTURE OF A COLLECTOR ASSEMBLY, A PLURALITY OF SHIELDS, EACH OF SAID SHIELDS DEFINING A RECESS, SAID SHIELDS BEING IN ELECTRICAL CONDUCTING RELATIONSHIP TO SAID DYNODES AND COLLECTOR ASSEMBLY, EACH OF SAID 